This invention relates to negative electrodes for use in batteries (e.g., lithium electrodes for use in lithium-sulfur batteries). More particularly, this invention relates to methods of forming alkali metal electrodes having a thin glassy or amorphous protective layer.
In theory, some alkali metal electrodes could provide very high energy density batteries. The low equivalent weight of lithium renders it particularly attractive as a battery electrode component. Lithium provides greater energy per volume than the traditional battery standards, nickel and cadmium. Unfortunately, no rechargeable lithium metal batteries have yet succeeded in the market place.
The failure of rechargeable lithium metal batteries is largely due to cell cycling problems. On repeated charge and discharge cycles, lithium xe2x80x9cdendritesxe2x80x9d gradually grow out from the lithium metal electrode, through the electrolyte, and ultimately contact the positive electrode. This causes an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. While cycling, lithium electrodes may also grow xe2x80x9cmossyxe2x80x9d deposits which can dislodge from the negative electrode and thereby reduce the battery""s capacity.
To address lithium""s poor cycling behavior in liquid electrolyte systems, some researchers have proposed coating the electrolyte facing side of the lithium negative electrode with a xe2x80x9cprotective layer.xe2x80x9d Such protective layer must conduct lithium ions, but at the same time prevent contact between the lithium electrode surface and the bulk electrolyte. Many techniques for applying protective layers have not succeeded.
Some contemplated lithium metal protective layers are formed in situ by reaction between lithium metal and compounds in the cell""s electrolyte which contact the lithium. Most of these in situ films are grown by a controlled chemical reaction after the battery is assembled. Generally, such films have a porous morphology allowing some electrolyte to penetrate to the bare lithium metal surface. Thus, they fail to adequately protect the lithium electrode.
Various pre-formed lithium protective layers have been contemplated. For example, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes an ex situ technique for fabricating a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride (xe2x80x9cLiPONxe2x80x9d) or related material. LiPON is a glassy single ion conductor (conducts lithium ion) which has been studied as a potential electrolyte for solid state lithium microbatteries that are fabricated on silicon and used to power integrated circuits (See U.S. Pat. Nos. 5,597,660, 5,567,210, 5,338,625, and 5,512,147, all issued to Bates et al.).
In both the in situ and ex situ techniques for fabricating a protected lithium electrode, one must start with a smooth clean source of lithium on which to deposit the protective layer. Unfortunately, most commercially available lithium has a surface roughness that is on the same order as the thickness of the desired protective layer. In other words, the lithium surface has bumps and crevices as large as or nearly as large as the thickness of the protective layer. As a result, most contemplated deposition processes cannot form an adherent gap-free protective layer on the lithium surface.
Thus, lithium battery technology still lacks an effective mechanism for protecting lithium negative electrodes.
The present invention provides an improved method for forming active metal electrodes having protective layers. Active metals include those metals that can benefit from a protective layer when used as electrodes. The method involves fabricating a lithium or other active metal electrode without depositing the protective layer on a layer of metal. Rather the lithium or other active metal is deposited on the protective layer. A current collector may also be attached to the lithium or active metal during the process.
One aspect of the invention provides a method of fabricating an active metal electrode, which method may be characterized by the following sequence: (a) forming a glassy or amorphous protective layer on a substrate; (b) depositing a first layer of active metal on the protective layer; and (c) providing a current collector on the first layer of active metal. The protective layer forms a substantially impervious layer which is conductive to ions of an active metal. In a preferred embodiment, the active metal is lithium and the protective layer is a single ion conductor which conducts lithium ions.
The substrate may be a sacrificial layer such as a releasable web carrier that includes a layer of copper, tin, zinc, aluminum, iron, etc. on which the protective layer is formed. Alternatively, the substrate may be a battery component such as a solid or gel electrolyte (e.g., a polymer electrolyte). After the electrode/electrolyte laminate is formed, it may be combined with a positive electrode and packaged to form a battery.
Preferably, the protective layer is formed on the substrate by a physical deposition process (e.g., sputtering) or a chemical vapor deposition process (e.g., plasma enhanced chemical vapor deposition). The alkali metal may also be deposited by a physical or chemical vapor deposition process. In one preferred embodiment, the active metal is an alkali metal that is deposited by evaporation.
The method may include affixing a current collector the remainder of the electrode. In one preferred approach, a second layer of the active metal is provided on the current collector (by evaporation for example). Then the current collector together with the second active metal layer is combined with the remainder of the electrode by bonding the second active metal layer to the first active metal layer (which is already affixed to the protective layer).
The invention also pertains to a partially fabricated battery cell which may be characterized by the following features: (a) a current collector; (b) a glassy or amorphous protective layer; (c) an active metal layer provided between the current collector and the protective layer; and (d) a gel or solid electrolyte provided on the protective layer opposite the alkali metal layer. Again, the protective layer forms a substantially impervious layer which is a single ion conductor conductive to ions of the active metal.
In one embodiment, the current collector is a layer of metal such as copper, nickel, stainless steel, and zinc. In another embodiment, the current collector is a metallized plastic sheet.
If the active metal is lithium, the protective layer should be conductive to lithium ions. Examples of suitable lithium ion conducting protective layer materials include lithium silicates, lithium borates, lithium aluminates, lithium phosphates, lithium phosphorus oxynitrides, lithium silicosulfides, lithium borosulfides, lithium aluminosulfides, and lithium phosphosulfides. Specific examples of protective layer materials include 6LiIxe2x80x94Li3PO4xe2x80x94P2S5, B2O3xe2x80x94LiCO3xe2x80x94Li3PO4, LiIxe2x80x94Li2Oxe2x80x94SiO2, and LixPOyNz (LiPON). Preferably, the protective layer has a thickness of between about 50 angstroms and 5 micrometers (more preferably between about 500 angstroms and 2000 angstroms). Preferably, the protective layer has a conductivity (to an alkali metal ion) of between about 10xe2x88x928 and about 10xe2x88x922 (ohmxe2x88x92cm)xe2x88x921.
The partially fabricated battery cell will generally be assembled into a completed primary or secondary battery. Examples of suitable primary batteries include lithium manganese dioxide batteries, lithium (CF)x batteries, lithium thionyl chloride batteries, lithium sulfur dioxide batteries, lithium iron sulfide batteries (Li/FeS2), lithium polyaniline batteries, and lithium iodine batteries. Examples of suitable secondary batteries include lithium-sulfur batteries, lithium cobalt oxide batteries, lithium nickel oxide batteries, lithium manganese oxide batteries, and lithium vanadium oxide batteries. Other batteries employing active metals other than lithium may be employed as well. These include the other alkali metals, alkaline earth metals (e.g., magnesium), and certain transition metals.
These and other features of the invention will be further described and exemplified in the drawings and detailed description below.